Series capacitor protection



April 18, 1950 R. E. MARBURY 2,504,350

SERIES CAPACITOR PROTECTION Filed July 20, 1946 .l lll l 7 l //a 6 9L f l m M I as 1 fil'yj a l g L H 9 7 EB mg; i Y II I I IL I /0 WITNESSES: INVENTOR ATTORN EY l atenteci Apr. l8,

SERIES CAPAUITQRPROTEGTIUN Ralph Marbiiry, r wilkinsburg; Pa; assignoito Westinghouse-Electric Corporatin,:-East {Pitts burgh; Pal, a'corporation if-Pennsylvania" Application J1il 26$1946f$fi91l No. 685,059"

16 Claims: (Cl. 175 -294) Thepresent invention relates tdseries capacitor protection and; more particularlyytda' protective systemfor series capacitors utilizing a' ther mal-ly responsive current carrying protective device."

Series capacitors are connectedinseries-in al ternating-current' distribution and transmission lin-esin "order'to neutralize p'art or all of the n'- ductive reactance of the Iine; and thfil's tb ilfi prove the" voltage regulation' or increase the* stability limitsbfs'uchlihes'f since series c'apaeitors *are connect'ed ih' series carry was line current, the "voltage across the capacitor is ie: portional t0 the 1in'e currfit,-' and the "capaci-tew may 'be subjected torelatively 'high overvolta'ges 5 incase of a fault, or air-abaerma-i-bverloaamnthe line. It is not "practical to utiliz'e capacitors' which 5 are capable of withstanding the-maximum voltage to whichthey may be 'subiectd h'rider fault con-(li tionspbecaiise' of their c0st-,-and for this "rea'sonit is necessary to provide protective means rr' by passing "the "capacitor :upo'n the "occurrence of an overvoltage.

Capacitor units ofthe" typefwhich are "used {as 1: series capacitors are capable -offwithstanding I voltage's ashigh szoo of the-rate'd voltag"for 5 verybr'ief periods, biit cannot-safely" be subj'ected to higher voltages even momentarily; Protective systems for 'series capacitors? therefore; usually operate to bY PaSS-thQ' capacitor 'upbn -tlfe' o'c currence of a voltage acrdsstne' apacimr corre'-fspo'nding'to 200 '--of "therated voltage of the capacitor units; The capacitor may be dam aged} however; by a lower overv'olta'g'e if it pe'r sists for "a sufficiently long time; ar'idit is often desirable to provide protec'tio'nio'r series "capaci tors against moderate-overvoltages which last for a long enoiigh time tobe-harmffilto the capacitor, 'asw'ell as substantially instantaneous protec tion against higher "overvolta ges. I}

A very eifective' protective device for"series" capacitors is disclosed and claimd in'aicopend' ing'application of "R. E; Marburyahd'J. BL Owen's; Srlal NO.5'72, 61 0; filed Januar ytw, 1945, and assigned to the Westinghouse Electric Corporation. Thisdevice consists essentially of a metallic -bel-' lows, and a switching means actuated--- by the" bellows and connected to complete aby-pass cir-" cuit around the protected-capacitor; The be'llows is connected to carry the-line'currentandto be heatedther'eby, and is normally-in a collapsed condition so astoholdthe switching means open; Upon the occurrenceof an excess-"current which heats the bellows toapredeterminedtemperature; the bellows expands,-and-operates, or permits=op--- 55 eration of, thesWitchir'ig means tb '"cbmplet' the bypass circuits tection; and thebellows can'be coh'nect'ii'in series with the capacitor to" afford "complete protection against moderate overvolt'ages;'sinceiVw-ill permit such ovei'voltages "to exist onthe capacitbr only fOi' a limited time, thetime beingashrter for higher voltages" than f 01 lower overvoltag'e's.

In this way: the capacitor is not taken" out of se'rvice"unlessthe overvoltage lasts long enoiighf to be" dangerous;- but it is positively protectedagainst moderataovervolta'ge's well as against higher excess Voltages;

If'the maxi-mum short ci'r'cuit" ciirr'ent of th line is not too high, such an' a'rrahgment'n'iay" be used alone? in connection "with" a capacitor which" is capable of withstanding the voltage correspondin'g'td'tlie-maxiniiiih"short circiiit ca'i rent for the brief time required f of th bellows to operate at this "current, since"the "eictra co'st of such -a capacitor is ofiSet'by the simplicity-and 10W cost bf theprotective-equipment. lf tfien lak imum short circuit cu'rfrent *is too high, coinpared t6 the nermar- 1inecurre'nt; additional 'in :1stantaneous prdtecti'on for higher oveivoltages"* may be provided-in the "usual "manner by means of a* spark gap connected across the capacitorg preferablywith a "second bellows-device in 'seiies with" the-"gap to" complete a "bypasscirciiit in re sponseto thegapcurrent."

When a -"-bellows device is connected "'in series with the capacitor," in'- the m'ann'er described above; and-carries the capacit'or current," its'*015- eratingcharacteristics miist*be coo'rd ihated 'with i the insulation characteristics of thecap'aciton'in' order to "provide proper pfotectidn, so thitth bellows- 'will always operate before*anoVerVOItage becomes dangerous to the capacitor, but war-net operate unnecessarily: Since 'the"-lii1e ciirrents andsystem characteristics*diffef v/idely in diifr' ent *ser ie's capacitor installations-" tills necessity for'coordination'of the bellows with''the'capacitoi"* makes it necessaryto"have' avai1able awide range of bellows devices having difierritciiirnt rat *ings,=* so that the proper bellows' can baselected for each installation; Thisyof-" course; is highly undesirable since it requires--a large 'nim'iher of difieren't de'sign's of bellows devices, having differ ent w'all thicknesses anddimensions, so as' 'to have "themec-ssary range'of current ratings; Inorde'r" to avoid this condition, it is desirable to provide an arrangement in which a single bellows device of standard current rating can be used in different series capacitor installations, so as to avoid the necessity of using a special bellows for each installation, thus making it possible to provide protective systems of this type for different series capacitor installations with the necessity of having avalable only one, or a very few, standard bellows devices of standardized current rating.

The principal object of the present invention is to provide a protective system for series capacitors utilizing a thermally-responsive currentcarrying device, such as a bellows, to actuate a bypassing means, in which a bellows of standard current rating can be used in different series capacitor installations so that only one, or a very few, standard types of bellows having standardized current ratings are required.

A further object of the invention is to provide a series capacitor protective system utilizing a current-carrying bellows device to actuate a bypassing means, in which the bellows is connected in series with an impedance device, which may be a part of the series capacitor bank itself, or which may be some other suitable impedance device, so that the bellows carries only a part of .the total line current, which can be adjusted by proper choice of the impedance device to permit the use of a bellows of standard current rating.

Another object of the invention is to provide a protective system for series capacitors utilizing a current-carrying bellows device to actuate a bypassing means, in which the series capacitor bank is divided into two or more parallel branches, with the bellows connected in series in one branch, so as to carry only a part of the total line current, which can be adjusted. by proper division of the capacitor bank into branches, to suit the current rating of the bellows.

A still further object of the invention is to provide a protective system for series capacitors utilizing a current-carrying bellows device to actuate a bypassing means, in which the bellows is connected in parallel with the capacitor, and an impedance device is connected in series with the bellows device to limit the current carried by the bellows to a value determined by its current rating so that a standard bellows device can be used in difierent series capacitor installations, the impedance device being either a resistor or an inductive device, and preferably having non-linear characteristics, so that it has high impedance under normal conditions but relatively low impedance under excess-current conditions to permit a large increase in the current through the bellows to obtain rapid operation.

The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawing, in which Figures 1 through 4 are schematic diagrams showing alternative embodiments of the invention.

Figure 1 shows a series capacitor installation connected in series in an alternating-current line I, which may be one phase conductor of a threephase line, or one conductor of a single-phase line, or, in general, any alternating-current line in which it is desired to introduce series capacitance. The series capacitor itself consists of a bank of at least two capacitors 2 and 3 connected in parallel. If desired, one or more additional capacitors 4 may be connected in parallel to the capacitors 2 and 3 in order to obtain the desired reactance and kva. capacity for the complete bank, but in this embodiment of the invention, at least two parallel-connected capacitors are required. Each of the capacitors 2, 3, or 4 may consist of a single capacitor unit, or of any necessary number of individual capacitor units connected together in any suitable manner, and the term capacitor as used in the specification and claims is to be understood in this sense.

The series capacitor bank, consisting of the parallel-connected capacitors 2 and 3, and any additional capacitors 4 that may be used, is protected against overvoltages by means of a protective device 5. The protective device 5 is shown diagrammatically as being of the type described in the above-mentioned Marbury and Owens application, and consists of a bellows device 6 and a switch I, which is actuated by the bellows device 6, and connected to complete a bypass circuit 8 around the entire capacitor bank when actuated to closed position.

The bellows device 6 is a metallic bellows capable of carrying current in order to be heated by the current. As more fully explained in the above-mentioned copending application, the bellows device 6 is substantially evacuated, and contains a small amount of a suitable liquid, such as water, which has a very low vapor pressure at temperatures corresponding to the current in the bellows under normal conditions, but which has a high enough vapor pressure to balance the atmospheric pressure and expand the bellows at a higher temperature at which operation of the bellows is desired. The bellows device 6 is connected to the switch 1 in any suitable manner, as by a mechanical linkage 9, to operate the switch, or to permit it to operate, to closed position when the bellows is expanded as a result of heating by an excess current. The bellows device 6 is connected in series with the capacitor 2, as shown, so that it carries only a part of the total line current, as determined by the relative reactances of the capacitor 2 and the parallel-connected capacitor or capacitors.

The operation of this system is as follows. Under normal conditions, the bellows 6, being substantially evacuated, is collapsed by atmospheric pressure, and the switch 1 is held open, so that the capacitor bank is in series in the line I. The bellows 6, which is in series with the capacitor 2, carries only a predetermined portion of the total line current, and this portion of the current may be matched to the current rating of the bellows 6 by a proper choice of the capacitors 2 and 3, to divide the total line current between them in a proportion which will give the desired current in the bellows 6. Upon the occurrence of an excess current, the temperature of the bellows will be increased, and when it reaches a predetermined temperature, the vapor pressure of the liquid in the bellows becomes high enough to balance the external atmospheric pressure and expand the bellows, so that the switch I is closed to complete the bypass circuit 3 and thus protect the series capacitor bank.

Since the bellows 6 is also bypassed by the bypass circuit 8, current ceases to fiow in it when the switch 1 closes, and the bellows commences to cool. An appreciable time is required for the bellows to cool down to its normal temperature, however, and this time is, in general, long enough for a fault or other disturbance on the line i to be cleared, so that when the bellows has cooled down sufficiently to be collapsed by atmospheric pressure and reopen the switch I to restore the capacitor bank to service, the overvoltage will usuaII W-IiavemasseG-E Incaseitheeexcess currenti-' slioul'd' 'still ;exist; fhowever;-w when the; switch 1".

opens; the *bellows. will immediately: start heat ing-:agaimand 'operatetmreclosee-theaswitch I in the sameemanner asabeforea It w'ill baseen; therefore,=- thatiaiprotective sysrtem is provided fomseriesrcapacitor installations:

moderate overvoltages which: can be permitted fora reasonabletirne; astwell 1 asagainst -higher overvoltages which 1 can be -permitted only for: a. very --brie -tir'ne By dividingethe*capacitor: bank into' -at least two parallel branches; withzthebellows' devic-e fi in series :with one off. thesbranches; so" as. to zcarr-y only: a=portionz of th totall' line; current; it'is possiblectofiuse a. standard bellows 6 having: astandardized current ratingiin dif+- ferent" series. capacitor installations; since. by. properly proportioning the relatives reactancesoi v the para-llel branches ofi the? bank; the" current invthe branch :in-which thesb'ellows 1 is connected: can :be matched tothe currcnt rating: of the bele lows; Thus,: the same bellowscan beusedin diff eren'tlseries capacitor installations: even though the -line=- currents. and other: conditions difier, widely. In this way, the necessity oftprovidinga large nurnber of difierent bellows of difierentcurrent ratings is avoided, and. protective" systems; of this type can beprovided... for difierent "in:- stallations with :asingle type: of: lo'ellowsfloiiv stand--' ard current rating;

The wsimpleprotective system described may bar-used alone *wherethe'maximum shortecircuit currentv of "the line is not xtoozgreatias;compared with the normal fullsload icurrent. If the-maximum--short-ecircuit.current' is not more:than three times the normal current} for example, .the. capacitor can be designed i; to: withstand; the corresponding overvoltagefor: the: brieftimer re-- quiredtforoperation of thesbellows. at thatvaliie oi current, and." the resulting-z increased: cost of the. capacitor is-offset by: thealow'costf and simplicity of the protective equipment. Where. higher -shortecircuit: currentszmay occur; additional instantaneous protection"against high overvoltages'- may: be provided by: means: of a spark: gapyor: in: any suitable manner.

Another embodiment"of'the-invention is shown iii-Fig: 2. This embodimentof."theainventionmti- 4, lizes the same: basic idea .of connecting thebe1-' lows in series with" an irnpedanceelement in par a11 e1with-th'e -capacitor bank; so-that'the bellows carries only a predetermined" part 0f the total current; which n can thus be ad'justed to match the currentrating of the -bellows; Fig. 2 shows a capacitor bankconsistingof any suitable-number.- of paralll connectedcapacitors a I 0,- connected in I series 'in the line l, and'=protected*-by a-protec-- tive device 5 identical witlrthat described'above in connectionr with 1 Fig: 12' In: this i embodiment o1 ethe invention, however, the ahellows fi is connected inseries'with' a resistor 'I I the series-connected=bel-lows t and-resistor: l I l beingiconnected in parallelwith the capacitors al 0; It will-be seen that with this arrangement; the-bellows 6- carries only a predetermined part of the" linecurrent which, bysuitable choice of the resistor I I, can:-

be-adjusted ito match:- th'e-curr.ent-' rating of i the bellows;

The-wresistor l I E mayzbeeoff'any suitablatype; butmreieraabl-yAs-ntithessofscallecbnomlinear type;

in which the efi'ective resistance: of the: resistor? decreaseswith increase in: current or voltage-v above a certaincritical value; Suchresistorsare' well' k nown in themselves, and may consistih-iors example, of granularsilicon?carbide moldedx to th'edesired size andshape within-suitable binder,

such as sodium silicate; and'b'akedi Resistors of this typehave relatively:- high: resistance under: low-voltages; but when the voltage :across there's sister; or the current through it, exceedsa certain critical value, .the'efiective resistance: rapidly :de-

creases -and' "permits heavy." currents to flow-i.

By using-a resistor :of this' type :in series; with the-*bel-lows-B; it is-possible to limitithe current;

through the bellows under normal conditions to a relatively low value, and the effectiveness-of the -series-capacitorin theline l is not significantly afiected by the high-resistance shuntl'coni sisting of tlieresistor- H andbellows fii Upon" the occurrence of a-fault on the: line I; or'otherabnormal condition; causing an excess current" to flow;theeffectiveresistance of the resistor l de:- creases, permitting a larger' proportionof the current-"to flow-through the -bellows -fi and rapidly heatingit to the r temperature" at which: it operates'to bypass the'series capacitori Thus, in this embodiment of-the invention, e ective and rapid protection of the series capacitoris obtained, and it also l'ias thesame advantage as the embodiment previously described. of permitting theuse' ofa standard bellows-in difierentseries capacitor installations; so that it is only necessary to have available one, or at most a very few, different types of bellowsoi'standardized currentratings;

A -much sharper increasein thecurrentthrough thebellows, uponthe occurrence ofan excesscurrent in the li'ne, can be obtained by the'use:

of asa-turablac iron-core reaction in series-with: thebellows; as shown in Fig. 3." The-embodiment of the invention-shown in this'fi'gu-re is identical with that of Fig. 2 except that the non-linear resistor is replaced b'y a reactor l5, having a saturable iron-core I6. The reactor: I5 is designed.

sothatvit isxsubstantially unsaturated.under::nor--- mal conditions' and. :has relatively high impedance;

to lii'nit the currentthroug-h the =be1lows- 6 Ito a relatively small rvaiue-,-. which is matched :to. the currentrating 'of.theabellows; When an excesscurrent-loccurs in the: line; I, the :increased cur-- rent :in thereactor li causes its: core IE: to sat-s. urate, resulting-ma largedecreasein the im pedance of v thezrea'ctor; and-:azsharp increase in. the current through the/bellows 6f, sonthatgrapida operation .Ofithe bellows is obtained." Thus, this; embodimentzof theinvention has the same ad 1 vantages as that of Fig. 2, but gives a much. sharper increase-".imthe: bellows. current under excessecurrent" conditions than. 1 can be obtained; with a monelineanzresistor;.

is shown in Fig.- 4.. In this: embodiment" of the.

invention, the bellows .6 .is .connectewacross. the series capacitor banki-in the samemanner-'asvin". Figs. 2f and :32 Theprimary winding ofiatrans former IT is :connectedin series with the bellows r.

the voltage across=-the secondary :winding of the Tfitransformer: l1" increases; and the sparks-gap. 18

Another type of non-linear impedance device which mayebeused .to control thezbellows' current;

breaks down, short-circuiting the secondary winding. This causes a large decrease in the effective impedance of the transformer l1, and results in a sharp increase in the bellows current and rapid operation of the bellows.

It will now be apparent that a protective system for series capacitors has been provided utilizing the thermally-responsive protective device disclosed in the above-mentioned Marbury and Owens application, in which the same bellows can be used in different installations, so that the difiiculty of coordinating the bellows device with the capacitor characteristics in different installations is avoided, and a standard bellows device can be used in series capacitor installations under widely varying conditions. In the embodiments of the invention shown in Figs. 2, 3 and 4, very rapid operation of the bellows is obtained by the use of non-linear impedance devices which permit the current in the bellows to increase more rapidly than the line current. It will be apparent that the invention is capable of various other embodiments and modifications. Thus, any suitable type of non-linear impedance device may be used, and if it is desired to provide instantaneous protection against higher overvoltages which cannot be permitted to occur across the capacitor even momentarily, a spark gap device may be connected across the capacitor bank in any of the embodiments shown, and any suitable means for by-passing the gap and capacitor may be provided in addition to the protective device 5, if desired.

It is to be understood, therefore, that although certain illustrative embodiments of the invention have been shown and described, it is not limited to the specific arrangements shown, but in its broadest aspects, it includes all equivalent embodiments and modifications which come within the scope of the appended claims.

I claim as my invention:

1. A series capacitor installation for an alternating-current line, said installation comprising at least two parallel-connected capacitors adapted to be connected in series in said line, and protective means for said capacitors, said protective means including a thermally-responsive device and switching means actuated by said thermally-responsive device, said switching means being connected to complete a bypass circuit around the capacitors when actuated by the thermally-responsive device, and the thermallyresponsive device being connected in series with one of said parallel-connected capacitors to be heated by a predetermined part of the line current.

2. A series capacitor installation for an alternating-current line, said installation comprising at least two parallel-connected capacitors adapted to be connected in series in said line, and protective means for said capacitors, said protective means including a bellows device adapted to carry current and 'to be heated thereby, switching means actuated by said bellows device, said switching means being connected to complete a bypass circuit around the capacitor when actuated by the bellows device, and the bellows device being connected in series with one of said parallel-connected capacitors to carry a predetermined'part of the line current.

3. A series capacitor installation for an alternating-current line, said installation comprising a capacitor adapted to be connected in series in said line, and protective means for said capacitor, said protective means including a thermallyresponsive device, switching means actuated by said thermally-responsive device, said switching means being connected to complete a bypass circuit around the capacitor when actuated by the thermally-responsive device, and an impedance device connected in series with the thermallyresponsive device, said impedance device being of a type in which the effective impedance decreases when the current increases, and said series-connected impedance device and thermally-responsive device being connected in parallel with the capacitor to carry a predetermined part of the line current.

4. A series capacitor installation for an alterhating-current line, said installation comprising a capacitor adapted to be connected in series in said line, and protective means for said capacitor, said protective means including a bellows device adapted to carry current and to be heated thereby, switching means actuated by said bellows device, said switching means being connected to complete a bypass circuit around the capacitor when actuated by the bellows device, and an impedance device connected in series with the bellows device, said impedance device being of a type in which the eiiective impedance decreases when the current increases, and said series-connected impedance device and bellows device being connected in parallel with the capacitor to carry a predetermined part of the line current.

5. A series capacitor installation for an alternating-current line, said installation comprising a capacitor adapted to be connected in series in said line. and protective means for said capacitor, said protective means including a thermally-responsive device, switching means actuated by said thermally-responsive device, said switching means being connected to complete a bypass circuit around the capacitor when actuated by the thermally-responsive device, and a resistor connected in series with the thermally-responsive device, said resistor being of a type in which the effective resistance decreases as the current increases, and said resistor and thermally-responsive device being connected in parallel with the capacitor to carry a predetermined part of the line current.

6. A series capacitor installation for an alternating-current line, said installation comprising a capacitor adapted to be connected in series in said line, and protective means for said capacitor, said protective means including a bellows device adapted to carry current and to be heated thereby, switching means actuated by said bellows device, said switching means being'connected to complete a bypass circuit around the capacitor when actuated by the bellows device, and a resistor connected in series with the bellows device, said resistor being of a type in which the effective resistance decreases as the current increases, and said resistor and bellows devicebeing connected in parallel with the capacitor to carry a predetermined part of the line current.

7. A series capacitor installation for an alternating-current line, said installation comprising a capacitor adapted to be connected in series in said line, and protective means for said capacitor, said protective means including a thermallyresponsive device, switching means actuated by said thermally-responsive device, said switching means being connected to complete a by-pass circuit around the capacitor when actuated by the thermally-responsive device, and an inductive impedance device connected in series with the thermany-responsive device, said impedance device :zbeing of avtype in which: the effective impedance decreases 'under excess-current conditions, and

:saidseries-connected impedance device, and

thermally-responsive device :being I connected in iparal'lel-with'the capacitor'to carry a predeter- =minedpartof the line current.

8. A series' canacitorinstal-lat on for an alter- :nating-current line, said installat on comprising a capacitor adapted to'beconnected in series in saidlineyandaprotective means for said capacitor, said protective means including a thermally-responsivedevice, switching means actuated by said thermally-responsive device, said switching means being 'connected'to complete a bypass circuitiaround the capacitor when actuated by the thermally-responsive device, and a saturable iron-core reactor connected in series with the thermally respons'ive device, "said reactor being adapted to saturate and reduce its impedance un'der excess-currentconditions, and said seriesc'onnected reactor and thermally-responsive de- -vice being connected in parallel with the capacitor to ca'rry a predeterm nedpart of the line current.

' 9. A series capacitor installation for an alternat ng=current line,"-said installation comprising a capacitor adapted to be connected in series in said line, and protective" means for said capac tor, said protective means 'incl'uding a conductive thermally-responsive device having inversetimecurrent characteristics, said thermally-responsive device be ng adapted to carry current and to be heated solely thereby, an impedance device connected in series with the thermally-responsive device, means for connecting said impedance device and thermally-responsive device in series with the line to carry a predetermined part of the line current, and switching means actuated by the thermally-responsive device, said switching means being connected to complete a bypass circuit around the capacitor when actuated, and the thermally-responsive device being adapted to efiect interruption of the bypass circuit after lapse of a time interval after actuation of the switching means.

10. A series capacitor installation for an alternating-current line, said installation comprising a capacitor adapted to be connected in series in said line, and protective means for said capacitor, said protective means including a conductive thermally-responsive device having inverse timecurrent characteristics, said thermally-responsive device being adapted to carry current and to be heated solely thereby, an impedance device connected in series with the thermally-responsive device, means for connecting said impedance device and thermally-responsive device in series with the line to carry a predetermined part of the line current, and switching means actuated by the thermally-responsive device, said switching means being connected to complete a by-pass circuit around the capacitor and the thermallyresponsive device when actuated, whereby current flow through the thermally-responsive device substantially ceases, and the thermally-responsive device being adapted to efiect interruption of the bypass circuit upon cooling due to the cessation of current flow.

11. A series capacitor installation for an alternating-current line, said installation comprising a capacitor bank adapted to be connected in series in. said line, and protective means for said capacitor bank, said protective means including a conductive thermally-responsive device having inverse time-current characteristics, said thermally-responsive device being adapted to -carrycurrent and to be heated solely thereby,

an impedance device connected in series with the thermally-responsivedevice, means for connecting said impedance device and thermally- -responsivedevice in parallel with at least a part of thecapacitor bank to carry a predetermined part of the line current, and switching means actuated by-the thermally-responsive device, said switching means being connected to complete a bypass circuit around the capacitor bank when and the thermally-responsive de- -12. A series capacitor installation for an alter- -nating-current line, said installation comprisdrug a capacitor bank adapted to be connected in series in saidline, and protective means for said capacitor bank, said protective means including a conductive thermally-responsive device having inverse time-current characteristics, said thermally-responsive device being adapted to carry current and to be heated solely thereby, an impedance device connec ed in series with the thermally-responsive device,means for'connecting said impedance device and thermallyresponsive device in parallel with at least'apart of the capacitorbank to carry a predetermined part of the line current, and switching means "actuated by-thethermally-responsive device, said switching means being connected to complete a bypass circuit around the capacitor bank and the thermally-responsive device when actuated, whereby current flow through the thermally-responsive device substantially ceases, and the thermally-responsive device being adapted to efiect interruption of the bypass circuit upon cooling due to the cessation of current flow.

13. Protective means for a capacitor connected in series in an alternating-current line, said protective means including a conductive thermallyresponsive device having inverse time-current characteristics, said thermally-responsive device being adapted to carry current and to be heated solely thereby, an impedance device connected in series with the thermally-responsive device, means for connecting said impedance device and thermally-responsive device in series with the line to carry a predetermined part of the line current, and switching means actuated by the thermally-respo-nsive device, said switching means being connected to complete a bypass circuit around the capacitor when actuated, and the thermally-responsive device being adapted to efiect interruption of the bypass circuit after lapse of a time interval after actuation of the switching means.

14. Protective means for a capacitor bank connected in series in an alternating-current line, said protective means including a conductive thermally-responsive device having inverse timecurrent characteristics, said thermally-responsive device being adapted to carry current and to be heated solely thereby, an impedance device connected in series with the thermally-responsive device, means for connecting said impedance device and thermally-responsive device in parallel with at least a part of the capacitor bank to carry a predetermined part of the line current, and switching means actuated by the thermally-responsive device, said switching means being connected to complete a bypass circuit around the capacitor bank when actuated, and the thermally-responsive device being adapted to effect interruption of the bypass circuit after lapse of a time interval after actuation of the switching means.

15. A series capacitor installation for an alternating-current line, said installation comprising a capacitor adapted to be connected in series in said line, and protective means for said capacitor, said protective means including a conductive bellows device adapted to carry current and to be heat-ed solely thereby, an impedance device connected in series with said bellows device, means for connecting the said impedance device and bellows device in series with the line to carry a predetermined part of the line current, and switching means actuated by the bellows device, said switching means being connected to complete a bypass circuit around the capacitor when actuated by expansion of the bellows in response to heating by an excessive current, and the bellows device being adapted to effect interruption of the bypass circuit when the bellows device cools.

16. A series capacitor installation for an alternating-current line, said installation comprising a capacitor bank adapted to be connected in series in said line, and protective means for said capacitor bank, said protective means including a conductive bellows device adapted to carry current and to be heated solely thereby, an impedance device connected in series with said bellows device, means for connecting the said imped- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 707,894 Andrews Aug. 26, 1902 1,718,980 Ringwald July 2, 1929 2,157,886 Cuttino May 9, 1939 2,202,715 Partington May 28, 1940 2,202,716 Partington May 28, 1940 2,323,720 Marbury et al. July 6, 1943 FOREIGN PATENTS Number Country Date 213,1 1 Switzerland Apr. 16, 1941 

